A plasma spray apparatus is employed for spraying powdered material to provide a permanent coating on all or part of a workpiece. This is done to modify the surface characteristics of the workpiece, for example, to change the dimensions of the workpiece, to make the workpiece more adaptable to temperature variations, to vary the coefficient of friction of the workpiece surface, to alter the ability of the workpiece to withstand exposure to abrasive chemicals and environment, etc.
A plasma spray apparatus utilizes powdered materials such as metals, ceramics, intermetallics or plastics. The powdered materials are carried to the plasma flame or stream by a powder feed system that utilizes a metering device and a feed tube. The powdered material is carried through the feed tube and toward the plasma flame by a pressurized gas. The powdered particles then pass through the plasma flame, where temperatures are typically in the range of 12,000 F. to 20,000 F., and are carried to and deposited on the workpiece.
Prior art plasma spray or powder feed systems require several seconds, typically eight or nine seconds, from the time the entire system is activated until a uniform flow of powdered material passing through the system is obtained. During this transition period, the rate of flow of powder will gradually increase from zero up to the desired rate for application on the workpiece. Similarly, when a prior art system is deactivated, several seconds elapse before the flow of powder stops completely. This elapsed or transition time is caused in part by the distance from the reservoir or hopper where the powder is stored to the workpiece. Additionally, the pressurized gas and the powder must overcome inertia to be accelerated during activation of the system, and decelerated during deactivation of the system.
This inertial phenomena of a variable flow rate during the transition period immediately following activation or deactivation of the prior art systems causes operating problems and inefficiencies. Specifically, workpiece specifications typically require the application of a uniform thickness of plasma spray powder. However, as mentioned above, the rate of flow of powder varies during the transition period following activation or deactivation of the plasma spray system. Therefore, to insure a uniform application of the powdered material on the workpiece, a prior art system must be activated and sprayed for several seconds before directing the plasma spray on the area to be coated. Similarly, the time required for deactivation of the prior art feed system cannot be commenced until after the plasma spray powder has been applied to the entire area for which the coating is required. As a result, the powder expended during the activation and deactivation periods is wasted causing a substantial inefficiency in the operation of the prior art plasma spray system. Furthermore, since the powdered materials used with plasma spray systems is extremely expensive, inefficiencies caused by the activation and deactivation periods adds significantly and unnecessarily to the cost of operating the system.
The magnitude of the loss caused by the elapsed or transition time in activating and deactivating a prior art feed system varies according to the workpiece. In certain applications, for example, it is desirable to apply a uniform thickness of plasma spray coating to one area of the workpiece without applying it to adjacent areas. To overcome the problem of variable flow rates during the transition period immediately after activation or deactivation of the system, a prior art plasma spray applicator would be directed at areas of the workpiece for which no coating is desired until the maximum flow is achieved. The spray then would be directed at the area to be coated, but then would be directed to another area for which no coating is desired during the deactivation period. A separate machining step would then have to be carried out to remove the plasma spray coating that was applied during the activation and deactivation of the prior art plasma spray system. As an alternative to machining the unwanted plasma spray coating of the workpiece, the workpiece could be initially masked adjacent to the area for which the coating is intended. Thus, the plasma spray would be directed at the masked area during the transition periods of powder feed. These machining and masking operations are time-consuming and add significantly to the manufacturing cost. Furthermore, as mentioned above, there is a substantial amount of costly powder wasted during the activation and deactivation, i.e., transition, periods when utilizing prior art feed systems in a plasma spray apparatus.
The magnitude of the inefficiencies described above is even greater in applications where a single workpiece includes several areas for which a uniform coating of plasma spray material is desired. Because of the time required to activate and deactivate the system, it becomes virtually impossible to shut the system down while moving the plasma spray applicator from one area of desired application to the next. As a result, the plasma spray system is operated at its peak application rate even in areas between the areas of desired application. Thus, the plasma spray material is applied at its full thickness in these areas and afterwards must be removed by machining or removal of the mask. In applications such as this, the amount of powder wasted may easily exceed the amount of powder applied.
Accordingly, it is an object of the subject invention to provide a plasma spray apparatus on which the flow of powdered materials may be started and stopped abruptly without a transition period of variable flow rate.
It is a further object of the subject invention to provide a plasma spray apparatus that avoids the need to machine or mask the areas adjacent to the parts of the workpiece on which the plasma spray coating is desired.
It is still a further object of the subject invention to provide a plasma spray apparatus that will significantly reduce the cost and time required to properly coat a workpiece.